To study the effect of charge transfer and chain stacking on the optical and thermal properties, a group of polyimides (PIs) were prepared from various diamines and dianhydrides containing sulfone, amide, and/or trifluoromethyl groups. Ultraviolet–visible spectroscopy, thermomechanical analysis, and thermogravimetric analysis were employed to study the properties of PIs. PIs containing sulfone and amide groups exhibited transmittance varying from 2.9% to 44.9% at 400 nm with good thermal stabilities of 5% weight loss in nitrogen at 455–466°C. The coefficient of thermal expansion values were 37.7–49.8 ppm/°C and glass transition temperatures were 261–299°C. The improved optical properties are owing to incorporation of amide group that decreases the electron-acceptability and sulfone group that distorts the chains, inhibiting the dense chain stacking. There is a trade-off relationship between the optical property and thermal property.
Spirobichroman-based polymers with high gas permeability and selectivity are promising for their applications as membranes in gas separation. In this study, three spirobichroman-based polyimides (PIs; 6FDA-FH, 6FDA-DH, and 6FDA-MH) were synthesised by the polyreaction between diamines containing different substituents (benzene ring, pyridine ring, and methyl group) and 4,4′-(hexafluoroisopropylidene)-diphthalic anhydride (6FDA). The physical properties, gas transport behaviour, d-spacing, dihedral angle of molecules, and fractional free volume of the PIs were investigated through experiments and molecular simulations. The PIs exhibited excellent thermal stability and good solubility in common organic solvents. The gas permeability of the PIs was investigated; the results highlighted the critical role of the substituents in the enhancement of the gas separation performance of polymer membranes. Detailed analysis of the PIs showed that 6FDA-FH exhibits the highest gas permeability. This can be ascribed to the loose packing of the polymer chain owing to the increased dihedral angle between the two planes. However, the methyl substituent in 6FDA-MH disrupts the polymer chain packing rather than changing the dihedral angle between the two planes, thus enhancing the gas permeability of 6FDA-MH. Furthermore, 6FDA-DH exhibited the highest CO2/CH4 selectivity, which is attributed to the CO2 affinity of the polymer containing the pyridine unit.
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